The present invention relates to a recording medium.
Recently, with progress of the laser technique, optical discs which utilize a laser beam to effect writing and reading of information have been put to practical use and extensive studies have been made thereon. Optical discs have such features as noncontacting operation, high recording density and stable preservation of recorded information. With these features, optical discs have first been used practically as a digital audio disc and a video disc both for reproduction only and then as a postscript type disc for document filing which permits the user to write information freely. Studies and developments have been made extensively also with respect to a rewritable type which permits the user to write and erase information freely in a repeated manner. And various proposals have been made on such rewritable type disc.
Inorganic materials such as tellurium compounds, amorphous rare earth elements, transition metals and magnetic alloys have been mainly used as recording materials in postscript type discs. However, these inorganic materials do not always satisfy the conditions required for recording materials, such as a superior memory characteristic, a high reliability and the capability of being mass-produced inexpensively. Also, a serious problem exists in point of toxicity. The use of organic materials such as organic coloring matters has also been tried actively which materials, however, have not been put to practical use yet because of inferior light resistance (resistance to ultraviolet rays). These materials, when used as recording materials, are each formed into a thin film on a substrate. But the formation of film requires such a technique as vapor deposition which, coupled with the high cost of the raw materials, causes an increase in cost of the resulting disc. Further, in most of discs made from those materials, small holes are formed at the time of recording by the melting of metal or the decomposition and evaporation of coloring matter. Consequently, a large quantity of heat is required for recording.
As rewritable optical recording materials, both inorganic systems such as Te-O-Ge-Sn, Sn-Te-Se and Ga-Te-Se systems and organic systems using photochromic materials, e.g. spiropyran, have been studied actively. However, the use of inorganic systems involves drawbacks such as the difficulty of attaining both a stable recorded condition and a high speed erasability as well as toxicity, difficulty of forming a thin film and a high cost. The use of photochromic materials also involves a serious problem that the repetitive stability is poor due to side reactions. Thus, neither organic nor inorganic systems have been put to practical use.